New applications of antennas for use in Radio Frequency Identification (RFID) devices are imposing difficult and controversial requirements on antenna specifications.
Antennas used in passive Radio Frequency Identification Systems deliver enough energy to the passive tag in a certain frequency band to power the tag, to transmit the required commands from the interrogator to the tag, and to receive the response from the tag in the form of a backscattered wave. In the case of passive tags, there is no other method of energy delivery beyond the antenna.
Typical operational frequencies include 433-435 MHz, 865-870 MHz, 902-928 MHz, 952-955 MHz, 2400-2500 MHz, 5700-5900 MHz and the level of the radiated energy is between 0.01 Watt EIRP and 4.0 Watt EIRP, and are defined and limited by the government regulations for RFID applications.
One feature of the passive RFID system, which distinguishes it from the other wireless communication systems, is that the transceiver transmits energy and information and receives backscattered signals from the tags at the same time and on the same frequency.
FIG. 1 illustrates a prior art RFID system, its components and signals. Similar to other wireless communication systems, the input port 10 of antenna 1 is connected to the transceiver 2 through the transmission line 3, which may be a coaxial cable, microstrip line, etc.
The transceiver transmits the signal with the power level Ptransmit to the antenna 1. Most of the transmitted energy will be radiated from the antenna into the space. A small portion of the transmitted energy will be reflected back from the not perfectly matched input port 10 of antenna 1 to the transceiver 2. The amount of this reflected energy is defined as return loss, or voltage standing wave ratio (VSWR), of the antenna.
Conventional wireless communication systems, in which the transmitted signal and the received signal are separated in time and/or by the frequency of the carrier, may employ antennas with VSWR in the range of about 1.5 to about 2.0, because more than 90% of transceiver power will be accepted by the antenna.
For RFID systems, such a value of VSWR may be too high, as the part of the noisy transmitted signal is coming back to the sensitive receiver. This will degrade the performance of an RFID system, where a noise floor is defined not by the noise figure of the receiver low noise amplifier (LNA) or the mixer, but by the portion of the signal from the transmitter that is “leaked” into the receiver.
Antennas preferably have a VSWR of less than about 1.20 for RFID applications. For instance, the reduction of antenna VSWR from 2.0 to 1.2 may increase the signal to noise ratio in the receiver by 11.3 dB. This increase in the signal to noise ratio may significantly reduce the errors of decoding of the signals coming from the tags and increase the speed of interrogation.
The portion of the power from the transmitter, which is accepted by the antenna, may be radiated into space with some correction in regard to the efficiency of the antenna. Radiated energy will be distributed in the volume of the space according to the radiation pattern of the antenna.
The interrogated tags 4, 5, 6 are usually positioned randomly in a volume of space. The maximum interrogation distance from the antenna to the tag is related to the minimum power that is needed to turn on or activate the integrated circuit of the tag, by the maximum power generated in the transceiver 2 of the RFID interrogator, and by the maximum realized gain of the antenna 1 and the radiation pattern of the antenna 1.
The maximum interrogation distance in free space is well defined, and is further described in “Friis Transmission Equation” (J. D. Kraus, Antennas, 2d Ed., McGraw-Hill, 1988, pp. 48-49). The radiation pattern of the antenna defines the maximum interrogation distances at angles other than at the boresight direction Z. Boresight is the optical axis of a directional antenna.
Traditionally the width of the radiation pattern is defined by the level where the gain of the antenna falls by 3 dB relative to the maximum gain. For a RFID system, such a definition is too coarse. With the 3 dB gain variation, the maximum interrogation distance will vary more than about 30%, estimated for free space with the “Friis Transmission Equation.” A more practical definition for RFID systems is that the maximum distance variation is less than 10%. This may be translated in the antenna gain variation as less than 1 dB. In FIG. 1, the angle θ represents the width of the pattern, where the realized gain of the antenna falls by 1 dB.
Most of the tags employed in RFID systems are electrical dipoles connected to the passive RFID integrated circuit. Such tags are sensitive to the polarization of the wave of radiated energy. If the polarization of the radiated wave will be linear the maximum interrogation distance will depend on the angle Φ between polarization of the radiated wave and the tag position. This maximum distance will vary from 0 to a maximum with Φ variations from 90° to 0°. That is why most of the typical RFID systems employ circular polarized antennas. To reduce the performance degradation of RFID systems, the antenna has to provide the variation of maximum interrogation distance of less than 10% for any angle Φ for a position of the tag relative to the antenna.
In terms of parameters of the antenna, the axial ratio of the antenna has to be less than 1 dB within the interrogation angle +/−θ from boresight axis Z.
Government regulations allow interrogation on the particular frequency or channel during a short period of time only. After that, the interrogator changes the frequency of the carrier or interrogation channel within the defined Frequency Band. That means that the antenna for RFID systems may provide radiation frequency bandwidth, within which the variation of the maximum interrogation distance is less than about 10%. Or, the radiation frequency bandwidth of the antenna may be defined as the antenna gain level at a point where it falls off by 1 dB relative to the maximum gain.
Another area of consideration of the antenna specification is if the antenna is portable and/or wearable. This imposes additional desirable characteristics, such as the minimum weight and the minimum size and/or volume of the antenna. Very often, the antenna is designed to fit into an existing portable device, with a position and a space to install the antenna defined before the antenna is designed.
Portable and/or wearable applications assume that the body of the user is close to the antenna and within a relative strong electromagnetic field. That means that the user will absorb some energy radiated from the antenna, and will provide some influence on the antenna parameters, such as VSWR, the Maximum Realized Gain and the Radiation Pattern. This point of view increases the value of the front to back ratio of radiation from the antenna.
At the present moment, many circular polarized antennas have been developed. But not one of them satisfies the characteristics and parameters presented above.
Full size circular polarized patch antennas with dimensions from (λ×λ×λ/16) to (λ/2×λ/2×λ/32), where λ is the free space wavelength of a radio frequency, provide relatively high realized gains of up to 8 to 9 dBic or 5 to 6 dBi. The size is defined by the conductive ground plane size and it is relative to the free space wavelength λ of radiated signal at the central frequency. The frequency bandwidth of such antennas is +/−3% to +/−3.5% at the −1 dB level from the maximum realized radiation gain. The front to back ratio is defined by the size of the ground plane and it is within from about 12 dB to about 18 dB, typically.
Polarization quality and/or axial ratio may be within about 1 dB, if the feeding circuit of the antenna employs a good quality 90° hybrid power divider. One drawback of these antennas is the size and the weight of the conductive ground plane. It is too large and heavy for most portable applications.
Small patch antennas with the small ground plane which employ a ceramic substrate with a high dielectric constant may be designed to fit into a volume defined by λ/5×λ/5×λ/32, or less. However, this type of antenna may be quite heavy, because of the high density of the ceramic substrate and it may possess a very narrow frequency bandwidth of about +/−0.6% or less from the central frequency at the level −1 dB from the maximum radiation gain. The front to back ratio may also be very poor, about 1 dB to about 2 dB only. Polarization quality and/or axial ratio also depends on surrounding objects and will be within from about 2 db to about 4 dB. The VSWR is typically above about 1.3 to about 1.5 within the frequency band.
Helical, helix and/or quadrifilar helix antennas do not use a conductive ground plane and because of this, the footprint may be much smaller than the footprint of patch antennas. A typical size of a quadrifilar helix is from λ/6×λ/6×λ/6 to λ5×/5×λ/2 or thicker. A thicker antenna provides a wider frequency bandwidth. An antenna with longer radiating elements wound around a cylinder provides a higher gain, but a narrower radiation frequency bandwidth.
The frequency bandwidth of such antennas is about +/−1.5% to about +/−2.5% at the −1 dB level from the maximum realized radiation gain. The front to back ratio is within from about 10 dB to about 16 dB typically. Polarization quality is very good and the axial ratio may be less than about 1 dB, if the feeding circuit of the antenna employs a good quality quadrature (0°, −90°, −180°, −270°) hybrid power divider.
One of the disadvantages of using a helical antenna is the length and/or thickness of the structure. The structure does not fit into small portable designs. Squeezing the thickness less than λ/6 by reducing the pitch will dramatically reduce the frequency bandwidth. The size may be reduced by employing the central core with a high dielectric material. However, using a high dielectric material increases the weight of antenna and reduces the radiation frequency bandwidth to about 1% or less and reduces the realized gain at the same time. See GeoHelix P2 Product Specification V6 Issue 11-06 Sarantel Ltd. (HQ) Unit 2, Wendel Point Ryle Drive, Park Farm South Wellingborough, NN8 6BA United Kingdom. See also U.S. Pat. No. 7,372,427 “Dielectrically-Loaded Antenna” & U.S. Pat. No. 6,886,237 “Method of Producing an Antenna,” which are hereby incorporated by reference.
Another disadvantage of using a helical antenna is the cylindrical shape. The radiation elements wrap around the supporting cylinder. If the cylinder is not perfectly shaped, such as being elliptical, it distorts the radiation pattern, increases the axial ratio of radiated field and the impedances of the radiation elements will be not equal, which will increase the VSWR at the input port. The pitch of the radiating elements or distance between them also impacts the operation as a little inaccuracy or instability can affect the operation. Sometimes, this inaccuracy can distort the pattern, can disbalance the impedances of the radiating elements, and can increase the VSWR. To produce the antenna with the radiating elements accurately wrapped around the cylinder, special technology is used other than the standard flat PCB manufacturing process adopted for patch antenna production. See U.S. Pat. No. 6,886,237 “Method of Producing an Antenna,” which is hereby incorporated by reference. This increases the cost of producing helical antennas relative to producing patch antennas.
Another type of circular polarized antenna is a crossed dipole antenna or turnstile antennas, as shown in FIG. 2. See U.S. Pat. No. 2,511,899 “Antenna System,” which is hereby incorporated by reference. See also J. D. Kraus, Antennas, 2nd Ed., McGraw-Hill, 1988, pp. 726-729. When this antenna has a good quality quadrature (0°, −90°, −180°, −270°) hybrid divider 8, it provides good polarization quality within a wide frequency bandwidth. The size of this antenna W is defined by the length of the dipole and is typically about λ/2, half of the wavelength in the free space.
The radiation pattern of the right hand circular polarized (RHCP) component is presented in FIGS. 3A and 3B. With phases 0°, −90°, −180°, −270° of the feeding signals at the radiating elements 7, the RHCP component propagates in the positive Z-direction. Unfortunately, the LHCP or cross-polarized component is as strong as the RHCP, but propagates in the opposite Z-direction. That is why the θ component of the electromagnetic field propagates in both positive and negative Z-directions. However, this is a waste of energy from the RFID system point of view.
The θ component of the electromagnetic field represents the component which provides the coupling with the linear polarized antenna of the RFID tag.
It is advantageous to concentrate the propagation in one direction only. One solution to accomplish this is to position the crossed dipoles above the conductive plane or inside of the conductive cup. See U.S. Pat. No. 3,740,754 “Broadband Cup-Dipole and Cup-Turnstile Antennas,” which is hereby incorporated by reference. But this solution increases the size of the antenna further and makes it less capable of being used in portable applications.
To reduce the size, some antennas proposed to bend the dipoles. See U.S. Pat. No. 6,211,840 “Crossed-Drooping Bent Dipole Antenna” & U.S. Pat. No. 4,686,536 “Crossed-Drooping Dipole Antenna,” which are hereby incorporated by reference. Such antennas have a smaller footprint, but the ground plane is still large and the overall structure is generally too thick for portable applications.
The way to reduce the footprint of the antenna further is to completely fold the dipoles 7 of turnstile antenna and position the dipoles 7 above the small size ground plane. The antenna appears as four monopoles above the small ground plane. See Lap K. Yeung et al. “Mode-Based Beam Forming Arrays for Miniaturized Platforms” IEEE Transactions on Microwave Theory and Techniques, January 2009, volume 57, pp. 45-52. This type of antenna system is presented in FIG. 4.
The radiating elements 7 are the thin strips of conductive material. These strips are positioned on the surface of thin dielectric substrates 9. The purpose of the dielectric substrate is to provide stability of radiating elements 7 in space. The combination of radiating elements and dielectric substrates may be produced through standard PCB manufacturing processes. The substrates 9 are assembled as a box, which provides a rigid, stable, and low weight structure. One end of the radiating elements is connected to the feeding circuit 8. The feeding circuit 8 divides the input signal into four signals with equal amplitude and phases (0°, −90°, −180°, −270°).
Such an antenna system poses the radiating patterns presented in FIGS. 5A and 5B similar to the conventional turnstile antenna. The RHCP component propagates in the positive Z-direction and the LHCP and/or cross-polarized component is as strong as the RHCP, and propagates in the opposite Z-direction.
The θ component has an almost omni-directional radiation pattern. For some applications it may be useful, but from a RFID point of view, it is a waste of energy, with loss of the antenna gain and reduced interrogation range.